This invention relates generally to a magnetic core of a transformer employed in inverters serving as a power supply system, and more particularly to such a transformer core wherein noise due to magnetostrictive vibration is reduced without worsening magnetic properties of the core.
Latest progress in electronic technology has found use of an increasing number of equipments each employing an inverter as a power supply system, for example, uninterruptive power supply units, vehicle transformers or rectifier transformers. In transformers employed in the inverters as the power supply system or rectifying transformers, an output voltage contains not only a fundamental wave component at the frequency of 50 or 60 Hz but a higher harmonic component in the frequency range between several 100 Hz and several 10 KHz. Noise due to the higher harmonic component is a critical problem. More specifically, noise in induction equipments results from magnetostrictive vibration in magnetic cores. The noise due to the magnetostrictive vibration is influenced by a power supply frequency. The noise is increased when the power supply frequency contains the higher harmonic component, which noise is offensive to the ears of persons. Accordingly, the induction equipments raise a problem of noise. Since some induction equipments, particularly, the uninterruptive power supply system are installed indoors, it is desirable that the noise be reduced in the induction equipments.
Conventionally, the transformer of the inverter employed as the power supply system or that of the rectifier includes a cut core formed by winding a number of grain-oriented 3% silicon steel sheets or a laminated core formed by laminating a number of grain-oriented 3% silicon steel sheets and a coil combined with either core.
However, the magnetostriction of the grain-oriented steel sheets employed in the above-described transformers takes a large value of 2.times.10.sup.-6 in the range of an operating magnetic flux density and accordingly, the higher harmonics due to on-off operation of a switching element forming a main circuit of the inverter increase the noise.
It has been proposed that 6.5% silicon steel sheets be employed to form the cores of the induction equipments such as the transformer, instead of the conventional 3% silicon steel sheets. The 6% silicon steel sheet has a low level of magnetostriction and superior magnetic properties. The 3% silicon steel sheet will hereinafter be referred to as "low content type silicon steel sheet" and the 6% silicon steel sheet as "high content type silicon steel sheet."
It has been known in the art that the level of the magnetostriction is nearly zero and the iron loss is reduced in the 6.5% silicon steel sheet. On the other hand, since the silicon steel sheet becomes fragile as the content of silicon is increased, it has been considered difficult to produce the silicon steel sheets by way of rolling. Recently, however, the 6.5% silicon steel sheets have been developed as the result of improvement in the technique for rolling fragile materials and development in the gaseous phase vapor deposition technique.
The 6.5% silicon steel sheet is a material advantageous in the noise reduction in the induction equipments since the magnetostriction thereof is an approximate value of 0.3.times.10.sup.-6. However, an exciting current is disadvantageously increased in equipments of commercial frequencies since the saturation value of the magnetic flux density is small in the low frequency range in the level of 50 to 60 Hz and the B-H (flux density vs. magnetizing force) characteristic is low in high flux density ranges. Accordingly, the magnetic flux density needs to be reduced when the 6.5% silicon steel sheets are employed in the equipments operated at the commercial frequencies but the reduction in the magnetic flux density renders the core large-sized, which prevents the equipments from being small-sized and light-weight. 0n the other hand, the B-H characteristic of the 6.5% silicon steel sheet in the magnetic high flux density range can be improved by setting the annealing temperature to a low value in the production of the 6.5% silicon steel sheets. In this case, however, the iron loss is increased.